Method and apparatus for coating metallic core with a metallic coating



Nov. 4, 1969 J. F. CLARKE ET AL 3,476,579

METHOD AND APPARATUS FOR COATING METALLIC CORE WITH A METALLIC COATING Filed Dec. 19, 1966 g i A H FREQUENCY Q ELECTRICAL I 2 POWER SOURCE United States Patent METHOD AND APPARATUS FOR COATING METALLIC CORE WITH A METALLIC COATING John F. Clarke, Atttleboro, and Clyde M. Adams, Jr.,

Lexington, Mass., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Dec. 19, 1966, Ser. No. 602,717 Int. Cl. B050 5/00; B44d ]/34 US. Cl. 117-51 7 Claims ABSTRACT OF THE DISCLOSURE A length of metallic core material is continuously moved first through a heating retort and then upwardly through a melt of metallic coating material capable of freezing on the core as the core leaves the melt. While passing through the retort, the core is surrounded by a protective atmosphere and has applied to it at two spaced points high-frequency alternating current which skin heats at a comparatively high surface temperature thereby efficiently cleaning its surface and providing for better surface wetting by the melt, but engendering only a small amount of heat energy. By heat conduction toward the center of the core this small amount of heat energy results in a comparatively low temperature of the core as a Whole when passing through the melt. Thus substantial amounts of metal are strongly and evenly frozen on the clean core without danger of remelt as the melt heats the core.

This invention relates to chill cladding and with regard to certain more specific features to the continuous chill cladding of a coating on core material in the form of a rod, wire, tube or the like.

Among the several objects of the invention may be noted the provision of rapid and low-cost means for chill cladding a coat of material (usually metal) on a metal core in the form of a rod, wire, tube or the like in a manner such that increased thickness of the coat can be clad on the core than heretofore obtained; the provision of means by which a better bond than heretofore can be obtained between the cladding and core materials as well as superior surface characteristics of the clad coat; and the provision of a stronger finished clad product. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the constructions, products and methods hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawing, the single figure diagrammatically illustrates one of various possible embodiments of the invention.

To avoid circumlocution, in the following the term substrate refers to a length of metal in the form of a wire, rod, tube, ribbon or the like of any of various cross sections, although only a wire is referred to by way of example in the following description of the invention.

It is known how to coat metal core material with a metallic cladding of another metal. For example, according to a former process, core material (for example, 0.125 inch of steel wire) was cleaned by immersing it in a conventional chemical cleaning solution, then preheating it to a predetermined temperature (for example, 900 F.) in a neutral :or reducing atmosphere and then immersing it in molten metal at a predetermined temperature, for example, aluminum at 1400 F.) for a predetermined length of time such that a predetermined amount of molten metal was solidified onto the solid core material. The cladding thickness of this former system was controlled fundamentally by the preheated temperature of the core, the cladding bath temperature and the residence time of the core in the bath. At the beginning of residence the rate of solidification or freezing of the coating on the core was considerable because of the large temperature gradient between them. However, the temperature gradient rapidly decreased as the bath heated the partially coated wire, with a tendency for the rate of freezing to decrease and even for the bath to remelt the initially solidified coat. The result was a severe limitation upon the amount of cladding that could be applied to the core material.

Other disadvantages of the former processes were the deleterious effects of improper cleaning of the core material and the fact that the hot coating initially came into contact with a relatively cold surface. These conditions resulted in inadequate initial wetting of the solid core material by the molten metal with consequent uneven and unpredictable heat flow. The result was considerable variation in bond strength and thickness of the as-cast cladding, both of which undesirably afiected the quality of the end product. The present invention overcomes these difficulties.

In high-frequency electrical heating, the current which produces the heat in the body being heated is concentrated in the outer skin portion of the body. The thickness of the skin in which the current is concentrated is represented by the equation:

-mf) where: =electrical resistivity (ohm-cm.) of the material, t=relative magnetic permeability (ohm-sec./cm.), f=frequency of the electrical power supply (c.p.s.),

S=skin depth (in which approximately of the current flows) in cm.

In the case of a carbon steel core where p is approximately 10- ohm-cm., and is approximately 1,000 ohmsec./cm., if a moderately high frequency such as 500 kilocycles is used, the current will be concentrated within an outer skin depth of approximately 10 microns, assuming a A: inch wire diameter.

Referring now more particularly to the drawing, numeral 1 indicates an upwardly moving continuous supply of high-tensile-strength Ms inch diameter steel wire which is to be clad with a coating of aluminum. An appropriate upward speed is feet per minute. Conventional means is employed for moving the wire and is represented by the dart 2. The aluminum is carried as a melt 3 in a crucible 5 through which the wire moves upward. The wire enters with a slip fit through an opening 4. Movement of the Wire through the aluminum forms a meniscus at 6. Thus downward leakage of the melt is inhibited. Attached to the crucible 5 under opening 4 is a preheating retort 7 through which the wire 1 passes before it enters the crucible. It enters retort 7 with a slip fit through an inlet 17 aligned with the inlet 4. The retort 7 has an inlet 8 and an outlet 10 for circulating a suitable inert or reducing atmosphere such as helium (inert), hydrogen (reducing), cracked ammonia (reducing) or the like all of which in the art are known as protective atmospheres.

Since opening 4 is not gas-tight, some of the atmosphere may reach the space under the meniscus 6. At numeral 9 is shown a source of A.C. high-frequency electrical power (500 kilocycles, for example). This high-frequency electrical power is applied to spaced points A and B on the wire 1 within the retort 7 over a circuit 11 and spaced brushes 13. A practical spacing l between A and B may be, for example, 3 inches. Point A, determined by the location of upper brush 13, should be as close as possible to the inlet opening 4. The high-frequency current flowing along the wire 1 between the brushes 13 will be in the skin of the wire. With a power input of 8 kilowatts, for

3. example, a desired skin temperature may be reached such as l250 F.1400 F. This is higher than the melting point of the aluminum in melt 3 which is about 1220" F. If this heat input into the skin were given time enough to spread throughout the entire volume of the wire between points A and B, it would heat it only to a temperature on the order of 900 F. Thus the frequency and power of the source 9, along with the speed of a wire 1 of a certain diameter and material, are selected so that first, a skin temperature (say 1250 F.-1400 F.) of the wire as it enters the melt 3 will be high enough so as first to effect good cleaning of the wire surface by evaporating contaminants and reduction of oxides by the reducing action of the atmosphere in retort 7; second, the skin temperature of the length l of the wire as it enters the bottom of the melt 3 will be considerably higher than the melting point (say, 1220 F.) of the aluminum melt; and third, during the residence time of an equal length the wire in the melt the heat from its skin shall have spread throughout its cross section and reduced its tem- I perature (900 F. for example) below the melting point of the melt.

As a result of the above, the initial contact ofthe melt the wire is on a very clean surface under excellent wetting conditions before any solidification or freezing of the melt on the wire. Thereafter as the wire ascends in the melt, the heat of the skin of the wire will spread throughout the mass of the wire, whereby the wire temperature will become reduced below that of the aluminum melt and below the melting point of aluminum by the time that the wire reaches its point of exit from the surface of the melt. This favors efiicient freezing onto the wire of a thick coat, without remelt. Thus it will be seen that an important feature of the invention is first to have the wire 1 enter the melt in a clean condition and at a skin temperature above the melting point of the melt and, second, to have its bulk temperature during a substantial time in the melt substantially below the melting point of the melt.

In view of the above it will be seen that a superior bond is obtained between the wire and the melt by reason of the superior wetting action between the melt and the clean hot surface of the wire. It will also be seen that remelting is minimized by delaying the reduction in the wire temperature for freezing action until the exit region of the wire from the melt is approached. As a result, thicker coating can be obtained than heretofore.

By only skin-heating the wire core at the high temperature in the retort 7, undesirable high-temperature annealing of the wire is greatly minimized, thus substantially maintaining its strength as originally manufactured.

The relationship for substantially maintaining the heat the heat in the skin portion of the wire until immersion in the melt is:

oat/#5001 where:

a=thermal diffusivity of the material (in. per sec.) r=radius of the wire in inches t=elapsed time in seconds If the wire is traveling at a velocity v between point A (close to inlet 4) and the point of freezing in the melt, the distance 1 between points A and B (in order to maintain the preheat in the skin) is given by:

where:

' l=the distance between points A and B in inches r=the radius of the wire in inches v=wire velocity in inches per second u=thermal diffusivity of the wire material in inches per second Employing the above formulations, for steel wire 0.250 inch in diameter, traveling at 100 feet per minute, the distance between the points A and B theoretically will be approximately 0.3 inch to maintain preheating in the skin. In practice, however, a certain amount of heat conduction from the surface of the wire to its inner bulk can be tolerated such that a uniform steep thermal gradient will be maintained in the Wire until the Wire enters the aluminum melt 3. Thus we have determined that in the lastmentioned example the distance 1 between points A and B may be extended to several inches (3 inches, for example) while still maintaining a satisfactory high skin temperature with substantial improvement in cleaning and wetting of the steel as it enters the bottom of the melt, and also obtaining sufficient temperature reduction and chilling capacity of the wire as it becomes totally immersed.

It will be understood that while the core material has been described above as being in the form of a wire, rod, tube or the like, it could have other shapes such as, for example, ribbon-shaped, with appropriate changes in the openings through which it passes to obtain sliding fits. In the following claims the term substrate will be used as a term generically covering all of the cross sections that the core material may have.

While it is preferred, as above described, that the skin temperature of the substrate 1 exceed the melting point of the melt 3 in order to provide improved wetting, some advantages other than wetting still accrue if this temperature is somewhat less than the melting point of the melt. Thus the temperature range of 12501400 F. mentioned above for the skin temperature might be in the range of 1200 F.1220 F.

The temperature range of the aluminum melt extends from a point sufiiciently above the melting point of aluminum to prevent inadvertent chilling to 1500 F. or above depending upon the cladding thickness desired.

It will be understood that the skin temperature may be higher than 1500 F., but no particular advantage is achieved by the additional heating.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods, constructions and products without departing from the gist of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative.

What is claimed is:

1. The method of chill cladding a melt of a metallic coating material on a length of electrically conductive metallic substrate material, comprising moving the length of the substrate material upwardly through the melt, passing a current of high-frequency electrical energy from an electrical source between two spaced points on the substrate material as it moves into the melt to heat and raise the skin temperature of the substrate material to a substantial degree higher than that of the remainder of the substrate, the heat energy supplied to the skin from said source being such that, upon its spreading throughout the bulk of substrate material when in the melt there will result a temperature of such substrate material which is less than the melting point of the melt.

2. The method of chill cladding a melt of a metallic coating material on a length of electrically conductive metallic substrate material, comprising moving the length of conductive substrate material through a retort containing a reducing or neutral atmosphere and then upwardly through the melt, passing a current of high-frequency electrical energy from an electrical source between two spaced points on the substrate material as it moves through the retort to heat and clean the substrate material and to raise its skin temperature to a degree higher than that of the melt, the heat energy supplied to the skin from said source being only such that, upon its spreading throughout a length of the bulk of substrate material in the melt, there will result a temperature of such substrate material which is less than the melting point of the melt.

3. The method according to claim 2, wherein the substrate material is a continuous length of steel and the coating material is aluminum.

.4. The methodaccording to claim 3, wherein the substrate material is in the form of a wire moving at a speed of approximately 100 feet per minute, wherein the skin temperature of the wire in the report is on the order of 1250 F.14()0 F., and wherein the temperature of the wire -in the melt is on the order of about 900 F.

5. Chill cladding apparatus comprising a crucible for containing a melt of metal coating material, said crucible having an inlet at its bottom, a retort therebeneath containing a protective atmosphere connected to said inlet, said retort having an inlet below and in alignment with said inlet of the crucible, means for moving a length of a metal substrate first through the retort and then upwardly through the melt in the crucible via said inlets, a source of high-frequency alternating electrical current, means for applying said current between two spaced points on said substrate in said retort, one of said points being adjacent the crucible inlet, the frequency of said current 'being such as to cause resistance-heating substantially only in the skin of the substrate while in the retort to an elevated temperature above the melting point of the melt and sufiicient to clean the surface of the substrate by evaporation of contaminants so as to remove them, the speed of the substrate being such that during its passage from the retort into the melt said elevated temperature will be substantially maintained until the clean substrate is wetted by the melt, the heat energy supplied to the skin by energy from said source being such that, upon its spreading throughout the bulk of a wetted length of the substrate in the melt equal to the distance between said spaced points, the temperature of said length will become reduced to a value below the melting point of the melt.

6. Chill cladding apparatus according to claim 5 wherein said protective atmosphere is inert.

7. Chill cladding apparatus according to claim 5 wherein said protective atmosphere is of the reducing type.

References Cited UNITED STATES PATENTS 2,216,519 10/1940 Quarnstrom 11751 X 2,320,801 6/1943 Simons 118-620 X 2,405,222, 8/ 1946 Mann 117-93 2,926,103. 2/1960 Brick.

2,937,108 5/ 1960 Toye 117-51 3,227,577- 1/ 1966 Baessler et al.

3,259,148 7/1966 Krengel et al. 117-51 X ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, IR., Assistant Examiner US. Cl. X.R. 

